Is "__module__" guaranteed to be defined during class creation?

Question

I was reading some code that looked basically like this:

class Foo(object):
    class_name = __module__.replace('_', '-')

To me, that looked really weird (__module__, what is that?) so I went and looked at the python data-model. A quick search shows that __module__ is a property of class objects and of function objects. However, there is no __module__ available in the global namespace (as can easily be verified by just trying to look at it and observing the NameError that results ...).

I decided to chalk this up to implementation specific behavior, but as a last check, I decided to test with other implementations I have handy. It turns out that this code executes with¹

• Cpython 2.7.6
• Cpython 3.4.0
• jython 2.5.3
• PyPy 2.2.1 (Python 2.7.3)

My question is whether this behavior is actually defined anywhere in the language reference. I'm not sure why I'd want to, but could I safely rely on __module__ being in the class creation namespace or did all the implementors just decide to do this the same way?

^{1All linux, but I doubt that matters ...}

Answer 1

What the documentation does define is that classes will have a __module__ attribute. It seems the way CPython does this is that it defines a local variable __module__ at the beginning of the class block. This variable then becomes a class attribut like any other variable defined there.

I can't find any documentation saying that __module__ has to be defined in this way. In particular, I can't find any documentation explicitly saying the attribute has to be define as a local variable in the class body, instead of being assigned as a class attribute at some later stage in class creation. This answer to a different question mentions that it works this way, and shows how it appears in the bytecode. There was a Jython bug that they fixed by making it work the same as CPython.

I'm guessing this is a CPython implementation detail that was carried over to other implementations. As far as I can tell the documentation doesn't actually say __module__ has to be available inside the class body, only on the class object afterwards.